Electroosmotic pump

ABSTRACT

Provided is a novel electroosmotic pump capable of being driven by AC voltage. An electroosmotic pump ( 2 ) includes a porous dielectric membrane ( 21 ), a first water-permeable electrode ( 22 ), and a second water-permeable electrode ( 23 ). The first water-permeable electrode ( 22 ) is disposed on one side of the porous dielectric membrane ( 21 ). The second water-permeable electrode ( 23 ) is disposed on the other side of the porous dielectric membrane ( 21 ). A principal surface of the porous dielectric membrane ( 21 ) close to the first water-permeable electrode ( 22 ) and a principal surface thereof close to the second water-permeable electrode ( 23 ) have mutually different hydrophilicities.

TECHNICAL FIELD

The present invention relates to electroosmotic pumps.

BACKGROUND ART

Recently, there have been increasing demands for micropumps which are a type of microfluidic device. The applications of micropumps are various, such as microreactors, hand-held medical devices, and fuel delivery for fuel cells. A mechanical micropump is conventionally known as a micropump. However, the mechanical micropump is composed of precision components. Therefore, the mechanical micropump is limited in cost reduction and size reduction. Against this background, attention is focused on an electroosmotic pump as a micropump to replace the mechanical pump (see, for example, Patent Literature 1).

Electroosmotic flow is liquid flow occurring when a voltage is applied to an electrical double layer where liquid and solid are in contact with each other. Electroosmotic flow has been found, together with electrophoresis, by the physicist Reuss in the early 19th century. In contrast to electrophoresis in which a solute or charged particles in liquid move, solid in the case of electroosmotic flow is immobilized. Therefore, when electroosmotic flow occurs, bulk liquid moves. Electroosmotic flow is observed in liquids composed of polarized molecules, including protic solvents, such as water and alcohol, ionic liquids, and so on. The electroosmotic pump is a pump configured to deliver a liquid using the electroosmotic flow.

CITATION LIST Patent Literature

Patent Literature 1: JP-A-2010-216902

SUMMARY OF INVENTION Technical Problem

Patent Literature 1 describes an example of the electroosmotic pump. In order to drive a conventional electroosmotic pump, such as the electroosmotic pump described in Patent Literature 1, it is necessary to apply a DC voltage thereto.

When a DC voltage is applied to the electroosmotic pump in order to activate it, an electrolytic reaction of the liquid concurrently occurs. When the electrolytic reaction of the liquid progresses, there arise problems, including a change in pH of the liquid and the generation of air bubbles in the liquid. Particularly when water is used as the liquid, this is dangerous because of the generation of hydrogen and oxygen. Therefore, a novel electroosmotic pump free of these problems is being strongly demanded.

A principal object of the present invention is to provide a novel electroosmotic pump capable of being driven by AC voltage.

Solution to Problem

A first electroosmotic pump according to the present invention includes a porous dielectric membrane, a first water-permeable electrode, and a second water-permeable electrode. The first water-permeable electrode is disposed on one side of the porous dielectric membrane. The second water-permeable electrode is disposed on the other side of the porous dielectric membrane. A principal surface of the porous dielectric membrane close to the first water-permeable electrode and a principal surface of the porous dielectric membrane close to the second water-permeable electrode have mutually different hydrophilicities.

In the electroosmotic pump according to the present invention, each of the first water-permeable electrode and the second water-permeable electrode is preferably a porous conductive film deposited on the surface of the porous dielectric membrane, a conductive mesh, a sintered film of conductive particles or a patterned electrode printed on a porous insulating film.

In the first electroosmotic pump according to the present invention, the porous dielectric membrane may include a hydrophilic layer on one of the principal surfaces.

A second electroosmotic pump according to the present invention includes a porous dielectric membrane, a first water-permeable electrode, and a second water-permeable electrode. The first water-permeable electrode is disposed on one side of the porous dielectric membrane. The second water-permeable electrode is disposed on the other side of the porous dielectric membrane. One surface of the porous dielectric membrane and the other surface of the porous dielectric membrane have mutually different zeta potentials or mutually different streaming potentials.

A third electroosmotic pump according to the present invention includes a porous dielectric membrane, a first water-permeable electrode, and a second water-permeable electrode. The first water-permeable electrode is disposed on one side of the porous dielectric membrane. The second water-permeable electrode is disposed on the other side of the porous dielectric membrane. The porous dielectric membrane is configured so that when an AC voltage is applied between the first water-permeable electrode and the second water-permeable electrode, a liquid in the porous dielectric membrane is given a force to selectively move from one of the side close to the first water-permeable electrode and the side close to the second water-permeable electrode to the other side.

In the first to third electroosmotic pumps according to the present invention, the porous dielectric membrane may include a first porous dielectric membrane and a second porous dielectric membrane which are stacked on each other, one of the principal surfaces of the porous dielectric membrane may be formed by the first porous dielectric membrane, and the other principal surface of the porous dielectric membrane may be formed by the second porous dielectric membrane.

Each of the first to third electroosmotic pumps according to the present invention may further include a power source operable to apply an AC voltage between the first water-permeable electrode and the second water-permeable electrode. In this case, the power source is preferably configured to apply an AC voltage with a frequency of 1 MHz or less.

In the first to third electroosmotic pumps according to the present invention, the porous dielectric membrane preferably has a thickness in a range of 5 μm to 100 μm.

In the first to third electroosmotic pumps according to the present invention, a ratio of the area of the first and second water-permeable electrodes to the square of thickness of the porous dielectric membrane ((the area of the first and second water-permeable electrodes)/(the thickness of the porous dielectric membrane)²) is preferably more than 100.

In the first to third electroosmotic pumps according to the present invention, the porous dielectric membrane preferably has an average pore diameter in a range of 10 nm to 50 μm.

In the first to third electroosmotic pumps according to the present invention, each of the first and second water-permeable electrodes preferably has a through hole passing through the water-permeable electrode in a thickness direction thereof.

In the first to third electroosmotic pumps according to the present invention, the porous dielectric membrane preferably has a through hole passing through the porous dielectric membrane in a thickness direction thereof.

In the first to third electroosmotic pumps according to the present invention, when the principal surface of the porous dielectric membrane close to the first water-permeable electrode has a higher hydrophilicity than the principal surface of the porous dielectric membrane close to the second water-permeable electrode, the first water-permeable electrode preferably includes a hydrophilic layer as a surface layer on the side opposite to the porous dielectric membrane.

Advantageous Effects of Invention

The present invention can provide a novel electroosmotic pump capable of being driven by AC voltage.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view of a liquid delivery module including an electroosmotic pump according to a first embodiment.

FIG. 2 is a schematic cross-sectional view of a portion of a liquid delivery membrane in the first embodiment.

FIG. 3 is a schematic cross-sectional view of a portion of a liquid delivery membrane in a second embodiment.

FIG. 4 is a schematic diagram of a hydrophilic layer of the liquid delivery membrane in the second embodiment.

FIG. 5 is a schematic cross-sectional view of a portion of a liquid delivery membrane in a third embodiment.

FIG. 6 is a photograph of a fracture cross-section of a track etched membrane used in Example 1.

FIG. 7 is a graph showing the relationship between applied voltage and flow rate in Example 1.

FIG. 8 is a graph showing the relationship between applied voltage and flow rate in Examples 1 to 3.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a description will be given of an exemplary preferred embodiment for working of the present invention. However, the following embodiment is simply illustrative. The present invention is not at all limited to embodiments below.

Throughout the drawings to which the embodiments and the like refer, elements having substantially the same functions will be referred to by the same reference signs. The drawings to which the embodiments and the like refer are schematically illustrated, and the dimensional ratios and the like of objects illustrated in the drawings may be different from those of the actual objects. Different drawings may have different dimensional ratios and the like of the objects. Dimensional ratios and the like of specific objects should be determined in consideration of the following descriptions.

First Embodiment

FIG. 1 is a schematic cross-sectional view of an electroosmotic pump according to this embodiment. FIG. 2 is a schematic cross-sectional view of a portion of a liquid delivery membrane in this embodiment.

A liquid delivery module 1 shown in FIG. 1 includes holding jigs 10, 11 and an electroosmotic pump 2 mounted to the holding jigs 10, 11. The electroosmotic pump 2 includes a liquid delivery membrane 20 sandwiched between a first water-permeable electrode and a second water-permeable electrode. AC power is supplied to the electroosmotic pump 2. The liquid delivery membrane 20 separates a first reservoir 12 and a second reservoir 13. The second reservoir 13 is connected to a liquid tank 30. Liquid is supplied from this liquid tank 30 to the first reservoir 12. The liquid supplied to the first reservoir 12 is delivered to the second reservoir 13 by the liquid delivery membrane 20 and then discharged through an outlet 14 provided in the second reservoir 13.

It is sufficient that the first reservoir 12 and the second reservoir 13 be those for guiding a liquid to one side and the other side of the electroosmotic pump 2 and providing a path along which the liquid is transported. The first and second reservoirs 12, 13 do not necessarily have a particular volume. The first reservoir 12 and the second reservoir 13 may be part of any flow channel of a microfluidic device. Furthermore, each of the first reservoir 12 and the second reservoir 13 may be filled with a water-permeable porous material or gel.

The liquid delivery membrane 20 may have a flat shape, a sagging structure, a structure with a plurality of asperities or a folded structure. In these cases, the ratio of the actual area of the surface of the liquid delivery membrane 20 to the area thereof in plan view ((the actual area of the surface of the liquid delivery membrane 20)/(the area of the liquid delivery membrane 20 in plan view)) can be increased. Therefore, the liquid delivery capacity of the electroosmotic pump 2 can be increased.

The liquid delivery membrane 20 includes a porous dielectric membrane 21. The porous dielectric membrane 21 is made of an appropriate dielectric material. The porous dielectric membrane 21 may be formed of, for example, a polymer membrane made of polycarbonate (PC), polyester (PET), polyimide (PI) or so on or an inorganic membrane made of ceramic, silicon, glass, sintered aluminum oxide, sintered aluminum nitride, sintered mullite, sintered silicon carbide, sintered silicon nitride, sintered glass-ceramic material or so on. Furthermore, the porous dielectric membrane 21 may be, for example, a porous monolithic material.

The porous dielectric membrane 21 is preferably a track etched membrane. The track etched membrane used herein means a membrane subjected to track etching. The track etching refers to chemical etching in which a membrane is irradiated with strong heavy ions to form linear tracks in the membrane.

When the porous dielectric membrane 21 is a polymer membrane or an inorganic membrane, pores can be formed therein by irradiation of laser light.

The porous dielectric membrane 21 is preferably a membrane with open cells and preferably a membrane with a plurality of through holes passing through the membrane in its thickness direction. Normally, the track etched membrane has a large number of through holes passing through the membrane in its thickness direction.

Although no particular limitation is placed on the thickness of the porous dielectric membrane 21, the thickness is preferably about 5 μm to about 100 μm and more preferably 10 μm to 60 μm. By setting the thickness of the porous dielectric membrane 21 at such a thickness, the thickness of the porous dielectric membrane 21 can be balanced with the thickness of an electrical double layer to be formed. Therefore, the electroosmotic pump 2 can be suitably operated.

The average pore diameter of the porous dielectric membrane 21 is preferably 10 nm to 50 m, more preferably 20 nm to 10 μm, and still more preferably 50 nm to 2 μm. If the average pore diameter of the porous dielectric membrane 21 is too small, the flow resistance may be large to make the amount of liquid delivered small. If the average pore diameter of the porous dielectric membrane 21 is too large, the hydraulic pressure of the delivered liquid may be reduced to deteriorate the energy efficiency of electroosmotic flow.

The porosity of the porous dielectric membrane 21 is preferably 1% to 50% and more preferably 3% to 30%. If the porosity of the porous dielectric membrane 21 is too high, adjacent pores are likely to merge with each other, which may present a problem with self-sustainability as a membrane. If the porosity of the porous dielectric membrane 21 is too low, the amount of liquid delivered may be small.

The pore density of the porous dielectric membrane 21 is preferably 4E2/cm² to 5E13/cm² and more preferably 3E4/cm² to 7.5E10/cm². If the pore density of the porous dielectric membrane 21 is too high, the porosity may be too high or the average pore diameter may be too small. If the pore density of the porous dielectric membrane 21 is too low, the energy efficiency of electroosmotic flow may be deteriorated.

A first water-permeable electrode 22 is provided on the side of the porous dielectric membrane 21 close to the second reservoir 13. A second water-permeable electrode 23 is provided on the side of the porous dielectric membrane 21 close to the first reservoir 12. It is sufficient that each of the first and second water-permeable electrodes 22, 23 be provided so that when a liquid is supplied, an electrical double layer is formed on the associated surface of the porous dielectric membrane 21. Each of the first and second water-permeable electrodes 22, 23 does not necessarily have to be in contact with the porous dielectric membrane 21. For example, conductive rubber with a high modulus of elasticity may be interposed between each of the first and second water-permeable electrodes 22, 23 and the porous dielectric membrane 21.

The first and second water-permeable electrodes 22, 23 are provided to allow liquid to permeate them in their thickness direction. Each of the first and second water-permeable electrodes 22, 23 preferably has through holes passing through it in its thickness direction. These through holes in the first and second water-permeable electrodes 22, 23 are preferably connected to the through holes in the porous dielectric membrane 21.

Each of the first and second water-permeable electrodes 22, 23 can be formed, for example, by depositing a conductive material, such as metal, on the porous dielectric membrane 21 so that the pores in the porous dielectric membrane 21 are not fully closed. Alternatively, each of the first and second water-permeable electrodes 22, 23 may be formed of, for example, a patterned electrode, such as a mesh electrode, a comb-shaped electrode, a staggered electrode or fractal patterned electrode.

Although no particular limitation is placed on the material for the first and second water-permeable electrodes 22, 23 so long as it is a conductive material, the first and second water-permeable electrodes 22, 23 are preferably made of a good conductive material. Specifically, each of the first and second water-permeable electrodes 22, 23 may be made of at least one metal of the group consisting of gold, silver, and copper, a composite material consisting predominantly of carbon, such as carbon nanotubes, a transparent conductive oxide, such as indium tin oxide (ITO), or so on.

The electroosmotic pump 2 includes an AC power source 40. This AC power source 40 applies an AC voltage between the first and second water-permeable electrodes 22, 23. The AC power source 40 preferably applies an AC voltage with a frequency of 1 MHz or less between the first and second water-permeable electrodes 22, 23, more preferably an AC voltage of 0.5 Hz to 20 kHz, and still more preferably an AC voltage of 1 Hz to 100 Hz. If the frequency of the AC voltage applied between the first and second water-permeable electrodes 22, 23 is too high, the electroosmotic pump 2 may not be suitably operated.

As shown in FIG. 2, in the electroosmotic pump 2, the porous dielectric membrane 21 includes a hydrophilic layer 21 a on a principal surface thereof close to the first water-permeable electrode 22. For example, when the porous dielectric membrane 21 is a track etched membrane, one of both surface layers of the porous dielectric membrane 21 is the hydrophilic layer 21 a. When the porous dielectric membrane 21 is not a track etched membrane, the hydrophilic layer 21 a can be formed by subjecting one of both surfaces of the porous dielectric membrane 21 to hydrophilic treatment as typified by plasma treatment, such as atmospheric plasma chemical treatment, or chemical modification with molecules having hydrophilic functional groups. An example of a polymer containing a hydrophilic functional group that is preferably used is polyurethane urea containing a phosphorylcholine group. Alternatively, examples of a polymer containing a hydrophilic functional group that can be used include polylysine and polyallylamine which have a large number of amino groups in their molecular chains. The method for chemically modifying the surface of the porous dielectric membrane 21 with molecules having hydrophilic functional groups is not limited to the above and any chemically modifying hydrophilic treatment technique that can be known by those skilled in the art is applicable.

The hydrophilicity of the surface of the hydrophilic layer 21 a is higher than that of a principal surface of the porous dielectric membrane 21 close to the second water-permeable electrode 23. Therefore, the principal surface of the porous dielectric membrane 21 close to the first water-permeable electrode 22 and the principal surface thereof close to the second water-permeable electrode 23 have mutually different zeta potentials or mutually different streaming potentials. Specifically, the zeta potential of the principal surface of the porous dielectric membrane 21 close to the first water-permeable electrode 22 is greater than that of the principal surface thereof close to the second water-permeable electrode 23 or the streaming potential of the principal surface of the porous dielectric membrane 21 close to the first water-permeable electrode 22 is greater than that of the principal surface thereof close to the second water-permeable electrode 23. Therefore, when an AC voltage is applied between the first water-permeable electrode 22 and the second water-permeable electrode 23, the liquid is transferred from the first reservoir 12 to the second reservoir 13. Thus, the electroosmotic pump 2 operates.

As just described, in this embodiment, the porous dielectric membrane 21 is configured so that when an AC voltage is applied between the first water-permeable electrode 22 and the second water-permeable electrode 23, the liquid in the porous dielectric membrane 21 is given a force to move from the side close to the first water-permeable electrode 22 to the side close to the second water-permeable electrode 23. Therefore, the electroosmotic pump 2 can be driven by an AC voltage. Hence, unlike the case where a DC voltage is applied to the electroosmotic pump, it is less likely that during drive of the electroosmotic pump 2 the liquid may be electrolyzed to change the pH of the liquid or change air bubbles.

The ratio of the area of the first and second water-permeable electrodes 22, 23 to the square of thickness of the porous dielectric membrane 21 ((the area of the first and second water-permeable electrodes 22, 23)/(the thickness of the porous dielectric membrane 21)²) is preferably more than 100. If the ratio ((the area of the first and second water-permeable electrodes 22, 23)/(the thickness of the porous dielectric membrane 21)²) is too small, the efficiency of liquid delivery becomes poor. There is no upper limit on this ratio.

The hydrophilicity can be measured with an automatic contact angle meter (DM-300, Kyowa Interface Science Co., Ltd.).

Zeta potential: The interface of solid or liquid in contact with a protic solvent, as typified by an aqueous solution, is electrically charged except for special cases. The electric field derived from the electric charges attracts ions of opposite sign (counter ions) from the solution side to form an ionic atmosphere (electrical double layer) near the surface. Specifically, there exist on the surface of the solid counter ions which are ions particularly strongly adsorbed thereto, and the electrical double layer includes: a Stern layer where counter ions are thus substantially immobile; and a diffuse electrical double layer having a structure in which counter ions are sparser with distance from the solid surface and are movably diffused. The zeta potential is a potential at a “slipping plane” (also referred to as a shear plane) located at the boundary between the Stern layer and the diffuse electrical double layer. The zeta potential at the membrane surface can be measured with, for example, a membrane zeta potential measurement system (ELSZ-1, Otsuka Electronics Co., Ltd.). The zeta potential in pores can be measured with, for example, a solid zeta potential measurement system (SurPASS, Anton Paar Japan K. K.).

The streaming potential can be measured with the solid zeta potential measurement system (SurPASS, Anton Paar Japan K. K.).

The electroosmotic pump of the present invention is operable by the application of an AC voltage thereto but does not necessarily have to be inoperable upon application of a DC voltage thereto. Normally, the electroosmotic pump of the present invention is operable not only upon application of an AC voltage but also upon application of a DC voltage.

Hereinafter, a description will be given of other exemplary preferred embodiments for working of the present invention. In the following description, elements having substantially the same functions as those in the first embodiment are referred to by the common references and further explanation thereof will be omitted.

Second Embodiment

FIG. 3 is a schematic cross-sectional view of a portion of a liquid delivery membrane in a second embodiment.

The electroosmotic pump according to this embodiment is different from the electroosmotic pump 2 according to the first embodiment in that the first water-permeable electrode 22 includes a hydrophilic layer 22 a as a surface layer on the side opposite to the porous dielectric membrane 21.

By the provision of the hydrophilic layer 22 a in the manner of this embodiment, the liquid delivery capacity can be increased.

For example, when the first water-permeable electrode 22 contains gold, the hydrophilic layer 22 a can be formed by subjecting the first water-permeable electrode 22 to surface treatment with a self-assembly reagent or the like capable of providing a gold-thiol bond. An example of the self-assembly reagent that can be preferably used is molecules with a main chain containing one end constituted by a sulfur atom and the other end constituted by a hydrophilic group. Specific examples of such a self-assembly reagent include:

HS—(CH₂)_(n)—COOH  (1);

HOOC—(CH₂)_(n)—S—S—(CH₂)_(n)—COOH  (2);

HS—(CH₂)_(n)—OH  (3);

HS—(CH₂)_(n)—(OCH₂—CH₂)—(CH₂)_(n)—OCH₂—COOH  (4);

HS—(CH₂)_(n)—NH₃Cl  (5); and

HS—(CH₂)_(n)—(OCH₂—CH₂)₆—NH₃Cl  (6).

FIG. 4 shows a schematic diagram of a hydrophilic layer 22 a formed using such a self-assembly reagent as described above (specifically, 1,1-mercaptoundecanoic acid).

Prior to the hydrophilic treatment with a self-assembly reagent, degreasing treatment, supercritical CO₂ cleaning, plasma treatment, corona discharge treatment or the like may be additionally performed.

Third Embodiment

FIG. 5 is a schematic cross-sectional view of a portion of a liquid delivery membrane in a third embodiment. As shown in FIG. 5, in this embodiment, the porous dielectric membrane 21 includes a first porous dielectric membrane 21A and a second porous dielectric membrane 21B. The first porous dielectric membrane 21A and the second porous dielectric membrane 21B are stacked. The first porous dielectric membrane 21A is located toward the first water-permeable electrode 22 and the second porous dielectric membrane 21B is located toward the second water-permeable electrode 23. The first porous dielectric membrane 21A is made of a material having a higher hydrophilicity than the second porous dielectric membrane 21B. Therefore, also in this embodiment, the surface of the porous dielectric membrane 21 close to the first water-permeable electrode 22 has a higher hydrophilicity than the surface thereof close to the second water-permeable electrode 23. Hence, the electroosmotic pump of this embodiment is also operable by the application of an AC voltage thereto.

The ratio between the thickness of the first porous dielectric membrane 21A and the thickness of the second porous dielectric membrane 21B ((the thickness of the first porous dielectric membrane 21A):(the thickness of the second porous dielectric membrane 21B)) is preferably 1:100 to 100:1 and more preferably 1:10 to 10:1.

The present invention will be described below in further detail with reference to specific examples, but the present invention is not at all limited by the following examples, and modifications and variations may be appropriately made therein without changing the gist of the invention.

Example 1

An electroosmotic pump having substantially the same structure as the electroosmotic pump 2 according to the first embodiment was produced in the following manner. A 20-nm-thick gold film was deposited on both surfaces of a track etched membrane with a thickness of 20 μm and an average pore diameter of 400 nm (Isopore membrane filter HTTP04700, Millipore) using a magnetron sputtering system (MSP-1S, Vacuum Device Inc.) to form a liquid delivery membrane. At that time, it was confirmed that the front and back sides of the membrane were electrically insulated. The first and second water-permeable electrodes made of gold were connected through respective conductive rubber electrodes to an AC power source. The distance between the first water-permeable electrode and the second water-permeable electrode was 20 μm equal to the thickness of the track etched membrane. FIG. 6 is a photograph of a fracture cross-section of the track etched membrane used in Example 1.

The zeta potential at the surface of the track etched membrane was measured using a membrane zeta potential measurement system (ELSZ-1, Otsuka Electronics Co., Ltd.). Specifically, the velocity of an electroosmotic flow induced by applying an electric field to and in parallel with the track etched membrane was observed as the velocity of motion of polystyrene latex (500 nm) made uncharged by modification with hydroxypropyl cellulose and the zeta potential was determined from the velocity of motion. A 10 mM NaCl aqueous solution was used as the liquid. The results are shown in Table 1 below.

TABLE 1 Zeta Potential Surface Close to First Surface Close to Second Water-Permeable Water-Permeable Electrode Electrode −10.46 mV −12.13 mV

The zeta potential in pores of the track etched membrane was measured by the streaming potential method using a solid zeta potential measurement system (SurPASS, Anton Paar Japan K. K.). As the results of calculation of zeta potentials from streaming potentials caused by application of a hydraulic pressure,

the zeta potential calculated from the streaming potential from the first water-permeable electrode toward the second water-permeable electrode was −36.01 mV, and

the zeta potential calculated from the streaming potential from the second water-permeable electrode toward the first water-permeable electrode was −40.11 mV. The zeta potentials in pores were much lower than the zeta potentials at the membrane surfaces.

A 25-Hz AC voltage was applied to the produced electroosmotic pump while the back pressure of the liquid (deionized water) was kept zero. The results are shown in FIG. 7.

In this example, substantially no air bubble was generated even when the AC voltage was continuously applied for 10 minutes.

The results shown in FIG. 7 reveal that the electroosmotic pump produced in this example is driven upon application of an AC voltage. The results also reveal that the flow rate can be increased by increasing the voltage applied.

Experimental Example 2

An electroosmotic pump was produced in the same manner as in Example 1 except that the surface of the first water-permeable electrode on the side opposite to the porous dielectric membrane was treated with 1,1-mercaptoundecanoic acid to form a hydrophilic layer.

Experimental Example 3

An electroosmotic pump was produced in the same manner as in Example 1 except that the surface of the first water-permeable electrode close to the porous dielectric membrane was treated with 1,1-mercaptoundecanoic acid to form a hydrophilic layer.

A 25-Hz AC voltage was applied to each of the electroosmotic pumps produced in Examples 2 and 3 while the back pressure of the liquid (water) was kept zero. The results are shown in FIG. 8, together with the results of Example 1.

The results shown in FIG. 8 reveal that the liquid delivery capacity can be increased by forming a hydrophilic layer on the surface of the first water-permeable electrode on the side opposite to the porous dielectric membrane.

Example 4

A liquid containing a pH indicator dissolved in deionized water was supplied to the apparatus produced in Example 1 and a 9.34-Vrms AC voltage was applied between the first and second water-permeable electrodes with 25 Hz for 15 minutes. Thereafter, the color tones of the first and second reservoirs were observed. The color tones of the first and second reservoirs were similar to those prior to the application of the voltage, their pH values were unchanged, and no gas due to electrolysis was generated. Also when a 0.9% by mass NaCl aqueous solution was used as a solvent, the first and second reservoirs exhibited no change in pH and no gas due to electrolysis was generated.

In contrast, when a 9.34-V DC voltage was applied between the first and second water-permeable electrodes for 15 minutes, the color tone of the first reservoir changed into an acid color, the color tone of the second reservoir changed into an alkaline color, and a gas due to electrolysis was generated. Furthermore, when a 0.9% by mass NaCl aqueous solution was used as a solvent, the color tone of the first reservoir changed into a strong acid color, the color tone of the second reservoir changed into a strong alkaline color, and a gas due to electrolysis was generated.

REFERENCE SIGNS LIST

-   1: liquid delivery module -   2: electroosmotic pump -   10: holding jig -   11: holding jig -   12: first reservoir -   13: second reservoir -   14: outlet -   20: liquid delivery membrane -   21: porous dielectric membrane -   21A: first porous dielectric membrane -   21B: second porous dielectric membrane -   21 a: hydrophilic layer -   22: first water-permeable electrode -   22 a: hydrophilic layer -   23: second water-permeable electrode -   30: liquid tank -   40: AC power source 

1-12. (canceled)
 13. An electroosmotic pump comprising: a porous dielectric membrane; a first water-permeable electrode disposed on one side of the porous dielectric membrane; and a second water-permeable electrode disposed on the other side of the porous dielectric membrane, wherein the porous dielectric membrane includes a first porous dielectric membrane and a second porous dielectric membrane which are stacked on each other, a first principal surface of the porous dielectric membrane is formed by the first porous dielectric membrane, a second principal surface of the porous dielectric membrane is formed by the second porous dielectric membrane, and the first principal surface of the porous dielectric membrane and the second principal surface of the porous dielectric membrane have mutually different zeta potentials or mutually different streaming potentials.
 14. The electroosmotic pump according to claim 13, further comprising a power source operable to apply an AC voltage between the first water-permeable electrode and the second water-permeable electrode, wherein the power source is configured to apply an AC voltage with a frequency of 1 MHz or less.
 15. The electroosmotic pump according to claim 13, wherein the porous dielectric membrane has a thickness in a range of 5 μm to 100 μm.
 16. The electroosmotic pump according to claim 13, wherein a ratio of the area of the water-permeable electrode to the square of the thickness of the porous dielectric membrane ((the area of the water-permeable electrode)/(the thickness of the porous dielectric membrane)²) is more than
 100. 17. The electroosmotic pump according to claim 13, wherein the porous dielectric membrane has an average pore diameter in a range of 10 nm to 50 μm.
 18. The electroosmotic pump according to claim 13, wherein each of the first and second water-permeable electrodes has a through hole passing through the water-permeable electrode in a thickness direction thereof.
 19. The electroosmotic pump according to claim 13, wherein the porous dielectric membrane has a through hole passing through the porous dielectric membrane in a thickness direction thereof.
 20. The electroosmotic pump according to claim 13, wherein the first principal surface of the porous dielectric membrane close to the first water-permeable electrode has a higher hydrophilicity than the second principal surface of the porous dielectric membrane close to the second water-permeable electrode, and the first water-permeable electrode includes a hydrophilic layer as a surface layer on the side opposite to the porous dielectric membrane. 